1. Field of the Invention
The present invention relates to a motor driving unit that that drives an object with the help of a driving force of an electric motor, and a vehicle-door mirror that is equipped with the motor driving unit.
2. Description of the Related Art
Motor driving units that drive objects with the help of driving forces of electric motors are know. Such motor driving units include a driving force transmission mechanism that transmits the driving force of the electric motor to the object, and a control circuit that control, such as switching off power supply to the electric motor, the electric motor. Such motor driving units are used in powered retractable door mirrors and power windows of vehicles.
A conventional motor driving unit is shown in FIG. 10. It is assumed here that,the motor driving unit is used in a powered retractable door mirror (hereinafter, “door mirror”) of a vehicle.
As shown in FIG. 10, the door mirror includes a mirror base 1, a shaft holder 2, a motor driving unit 3A, and a mirror assembly that is the object to be driven by the motor driving unit 3A. The mirror base 1 is fixed to a door (not shown) of a vehicle. The shaft holder 2 holds a shaft 21 and is fixed to the mirror base 1. The motor driving unit 3A is rotatably pivoted by the shaft 21.
The mirror assembly includes a housing 4, a fixing bracket 5 that includes a power unit etc., and a mirror 6. The mirror assembly is rotatably pivoted by the shaft 21 via the motor driving unit 3A.
The motor driving unit 3A includes an electric motor 33, a clutch mechanism 35, and a speed reduction mechanism (driving force transmission mechanism) 36. The clutch mechanism 35 transmits the driving force of the electric motor 33 to the mirror assembly. The motor driving unit 3A further includes a switching circuit (motor drive control circuit). When the mirror assembly comes to a predetermined position, the switching circuit switches off the power supply to the electric motor 33 and stops the mirror assembly at a standing position (in-use position) or a retracted position (rearward tilted position).
As shown in FIG. 11, the switching circuit includes a first PTC (positive temperature coefficient) element PTC1, a second PTC element PTC2, a first diode D1, a second diode D2, a relay contact R-S, a start-up condenser C, a relay self-holding resistance R-R, a relay-coil R-C, and a discharge resistance RD.
The switching circuit is connected electrically to a power supply ACC (+) side and an earthing (−) side via a retraction control switch (not shown) and a harness (not shown). The retraction control switch is for switching to the standing position or to the retracted position, and is disposed in an interior of the vehicle.
When an operator operates the retraction control switch, the motor driving unit 3A is operated (by driving the electric motor 33 and operating the speed reduction mechanism 36 etc.) and the mirror assembly is moved to the standing position or the retracted position. When the mirror assembly reaches the standing position or the retracted position, the switching circuit switches off the power supply to the electric motor 33 so that the mirror assembly stops at that position.
Generally the electric motor 33 produces a high-frequency induced-noise due to a change in the amount of electric current flowing during driving (a momentary voltage fluctuation at a start-up of drive, while switching from normal rotation to reverse rotation etc.). The induced noise affects peripheral equipments. For example, if the motor driving unit is used in functional parts of the vehicle such as the powered retractable mirror and the power window, the induced noise affects equipments in the interior of the vehicle such as radio, television, and electronic control unit (ECU) that performs an engine control etc. Moreover, if the motor drive control is used as a switching device of top light in a house, it affects equipments such as radio and television in the house.
One approach to avoid the induced noise from affecting the peripheral equipment is to enclose the peripheral equipment in a casing that blocks the induced noise.
However, induced noise not only from the electric motor but also induced noises from various sources affect the peripheral equipment. For example, the relay contact R-S also produces noise. Therefore, it is necessary to block these induced noises that are in various frequency bands. The conventional technology focuses only on blocking the induced noise form one source, that is, the electric motor.